Conventional gas turbine engines are enclosed in an engine case and include a compressor, a combustor, and a turbine. Air flows axially through the sections of the engine. The air, compressed in the compressor, is mixed with fuel which is burned in the combustor and then expanded in the turbine, thereby rotating the turbine and driving the compressor.
The compressor includes alternating rows of rotating airfoils or rotor blades and stationary airfoils or vanes. Each rotor blade has a leading edge and a trailing edge extending from a root of the blade to a tip thereof and a pressure side and a suction side. Each rotor blade is secured onto a rotor disk. Each row or stage of airfoils contributes to the compression of the air. Thus, the pressure of the air upon leaving each stage at the trailing edges of the blades is higher than the pressure of the air upon entering each stage at the leading edges of the blades. Also, the pressure side of the blade has higher pressure air than the suction side thereof.
A problem arises when a relatively large tip clearance exists between the tips of the compressor rotor blades and the engine case. The tip clearance allows the higher pressure air from the pressure side of the blades to leak into the lower pressure suction side area of the blades. The leakage causes inefficiencies in the gas turbine engine performance, because the higher pressure leaked air must be compressed again, thereby requiring the compressor to perform some portion of work more than once.
The size of the tip clearance varies with the operating conditions of the gas turbine engine and is associated with different amounts and rates of expansion and contraction of the engine case and the rotor assembly. The expansion and contraction of the engine case is a function of the pressure and temperature, whereas the expansion and contraction of the rotor and blade assembly is affected by centrifugal force and the temperatures of the rotor and the disk within the compressor. Also, the engine case and the rotor assembly are fabricated from different materials, each having different coefficients of expansion. The comparative mass of the rotor assembly and the engine case is another contributing factor to the variations in the tip clearance during transient stages of the engine operation. Since the rotor assembly has greater mass than the engine case, it takes a longer time to heat the rotor assembly than it does to heat the engine case. Consequently, the engine case expands faster than the rotor assembly.
As the gas turbine engine begins to operate, the rotor expands almost immediately due to the centrifugal force, reducing the tip clearance. Then, the engine case expands due to the increase in pressure, thereby increasing the tip clearance. The amount of expansion of the engine case due to the increase in pressure is different from the amount of expansion of the rotor assembly. Subsequently, the engine case is subjected to thermal expansion due to increased temperature, further increasing the tip clearance. The rotor and blade assembly also expands thermally due to increased temperature, reducing the tip clearance. The rate of thermal expansion of the rotor assembly is slower than the rate of thermal expansion of the engine case, because the rotor assembly is much heavier than the engine case, and therefore, the rotor takes a longer time to heat up. Hence, the tip clearance between the tips of the blades and the engine case changes non-uniformly, frequently resulting in a relatively large gap that allows leakage of higher pressure air to the lower pressure air area, thus resulting in engine inefficiency.
In one attempt to minimize tip clearance, conventional gas turbine engines use an abradable liner within the engine case. The tips of the rotor blades make contact with the abradable liner, carving out the material therefrom. At a certain point of operation, the tip clearance will be zero, but at all other points of operation, there will be a gap between the tips of the rotor blades and the liner caused by the removal of material by abrasion, allowing the undesirable leakage of higher pressure air into the lower pressure air area. An additional problem with abradable liners is that during hard landings or airplane turns the rotor deflects differently than the engine case. As a result, the rotor blades carve out additional material from the abradable liner, thereby enlarging tip clearance permanently.
Another approach used to minimize tip clearance is to fabricate a greater mass engine case to more closely match the effective thermal expansion rate of the greater mass rotor and blade assembly. Although this approach minimizes the tip clearance during some operating conditions of the gas turbine engine, it increases the tip clearance at idle. Also, this approach results in an undesirable increase in the overall weight of the engine.
Another solution used to reduce the tip clearance is to eliminate thermal mismatch between the engine case and the rotor assembly. This effect is achieved by pumping hot or cool air around the case to correlate thermal expansion and contraction of the engine case with that of the rotor and blade assembly. There are a number of drawbacks associated with this procedure. First, the procedure requires expensive, complicated hardware to control the thermal expansions and contractions. Second, the additional hardware results in a weight penalty. Finally, the approach requires bleeding hot and cool air from the engine, thereby resulting in inefficiency.
Thus, currently there is still a great need to effectively minimize tip clearance between the tips of the rotor blades and the engine case liner.